Tribologically Loadable Mixed Noble Metal/Metal Layers

ABSTRACT

The invention relates to a method for producing a noble metal/metal layer, which has particularly advantageous tribological properties, comprising the following steps: providing a bath for the currentless deposition of a metal layer, which additionally contains at least one type of noble metal ions; introducing a substrate into the bath; and applying a voltage.

FIELD OF THE INVENTION

The invention relates to a method for producing tribologically loadable noble metal/metal layers. These are layers having a thickness of up to 50 μm. The invention also relates to substrates having such a coating and the use thereof.

PRIOR ART

Numerous metal layers containing noble metals are known from the prior art. These layers typically consist of at least one noble metal in a mixture or alloy with at least one metal which is baser in comparison to the noble metal. Such layers allow the properties of the base metal layer to be improved. This improvement can, in this case, be in the corrosion resistance, hardness, conductivity, or the biocidal properties. Simultaneously, a lower noble metal content makes the production of the layers more cost-effective. The layers also partially maintain the advantageous properties of the base metal layers.

The layers can be deposited either using galvanic or currentless methods. In the currentless methods, noble metal ions are typically added to the known baths for the currentless deposition of a metal. Since the noble metal may be reduced significantly more easily, it is deposited together with the metal as a mixture. However, it is difficult to obtain layers having a high content of noble metal, in particular tribologically loadable layers, using such methods.

In galvanic methods, corresponding solutions of metal salts and noble metal salts are deposited under voltage from a bath. However, due to shielding effects it is not possible to uniformly coat cavities or cavities enclosed by nets, as occur for example in filter elements, and in particular to resolve fine structures, e.g., structural elements in the range of less than 200 μm, with precise contours, using galvanic methods.

Pure noble metal layers, which are deposited without current, are often soft and do not display sufficient abrasion resistance. Contour resolution or accuracy and throwing power are not sufficient for complete and uncorrupted coverage of fabric or gap structures. In point of fact, hard metals (e.g., cobalt) may be galvanically co-deposited to form a higher abrasion resistance. The layers are not nonporous and are soon infiltrated or detached.

The publication DE 10 2006 020 988 A1 describes the production of nickel layers containing noble metals, for example.

The publication GB 1 222 969 describes the galvanic deposition of a metal from a bath, which is also suitable for the currentless deposition of the same metal.

It is not possible using most methods to achieve high proportions of the noble metal in the layer and simultaneously to obtain good tribological properties of the coatings. This is true in particular for screens and nets, in which the tribological properties play a special role. This is significant in particular in the case of filter elements for filtering fluids.

OBJECT

The object of the invention is to overcome the disadvantages of the prior art and to provide a method, using which metal layers containing noble metals can be obtained, which have advantageous tribological properties in particular. The method should also allow nets or screens to be coated. The layers must be produced as nonporous in particular, to prevent infiltration, in particular in the case of use in fluids.

SUMMARY OF THE INVENTION

This object is achieved by the inventions having the features of the independent claims. Advantageous refinements of the inventions are characterized in the subclaims. The wording of all claims is hereby made part of the content of this description by reference. The invention also comprises all reasonable and in particular all mentioned combinations of independent and/or dependent claims.

The object is achieved by a method for depositing a noble metal/metal layer on a substrate, which comprises the following steps:

a) providing a bath for the currentless deposition of a metal layer, which additionally contains at least one type of noble metal ions; b) introducing a substrate into the bath; c) applying an electrical voltage.

Individual method steps are described in greater detail hereafter. The steps need not necessarily be carried out in the specified sequence, and the method to be described can also comprise further steps which are not mentioned.

A noble metal/metal layer is a layer in which the proportion of noble metal in wt.-% is higher than the proportion of metal. The reverse is true for a metal/noble metal layer.

Firstly, a bath for the currentless deposition of a metal layer, which additionally contains at least one type of noble metal ions, is provided. Such baths for currentless deposition are known to a person skilled in the art. This is a bath which consists, e.g., of an aqueous solution of a salt of a metal, which is to be deposited on the substrate by reducing the salt. The reduction is performed without current by a reducing agent. In this case, deposition typically does not begin until specific conditions, typically selected from pH value and/or temperature of the bath, have been achieved. Such currentless methods are frequently autocatalytic systems. This means that the deposited metal layer catalyzes the further deposition of metal. The thickness of the deposited layer can be controlled via the duration of the deposition.

Many metal/reducing agent systems for currentless deposition are known to a person skilled in the art. Examples of metals deposited without current are nickel, copper, palladium, silver, or gold. Aldehydes, e.g., formaldehyde, formic acid, borohydride compounds, for example, alkylamine boranes, dimethyl and diethyl aminoborane or sodium borohydride, also hydroxylamine, hydrazine, hydroxycarboxylic acids, the salts thereof, or thiourea or the derivatives thereof, phosphorus compounds, for example, hypophosphites such as sodium hypophosphite, can be used as reducing agents. A reducing gas, such as hydrogen, can also be conducted through the bath.

Examples of metal/reducing agent systems are copper/formaldehyde, gold/formaldehyde, palladium/hypophosphite, silver/hypophosphite, nickel/borohydride, and nickel/hypophosphite.

The proportions of metal and reducing agent in the bath are dependent on the metal and reducing agent used. The proportion of metal can thus be between 0.01 and 20 g/l and the content of reducing agent can be between 5 and 50 g/l.

The solvent of the bath is preferably water. However, organic solvents can also be used or added, if the solubility of the bath components in water is not sufficiently high. The organic solvents can also be added proportionally. In particular lowmolecular-weight alcohols come into consideration as organic solvents. Preferably, only water is used as the solvent.

The corresponding chlorides, sulfates, carbonates, acetates, or nitrates are typically used as the metal salts. Mixtures of metal salts with different cations and/or anions can also be used.

The bath can also contain still further additives. For example, oxocarboxylic acids or complexing agents, which prevent the decomposition of the bath.

The bath can also contain complexing agents for the ions of the metal salts to reduce the quantity of free metal ions. Depending on the metal salt used, these can be carboxylic acids, amines, alkyl amines, amino acids, phosphonates, cyanates, isocyanates, thiocyanates, ethers, or thioethers. Examples of such compounds are citric acid, chelate ligands such as ethylene diamine tetraacetic acid, 1,3-diaminopropane, 1,2-bis-β-(aminopropylamino)ethane, 2-diethylaminoethylamine, and diethyllene triamine, or polyethylene glycols.

The bath additionally contains at least one type of noble metal ions. Noble metal ions are ions for metals which, according to the electrochemical series, have a greater reducing potential than the other metal salts for currentless deposition in the bath. The noble metal ions are preferably selected from the group containing silver, gold, palladium, platinum, rhodium, and copper. In the case of silver, the biocidal effect of silver-containing surfaces can additionally be utilized.

The noble metals are preferably added as salts or solutions of their salts. In this case, chlorides, sulfates, carbonates, acetates, nitrates, sulfonates, sulfites, alkyl sulfonates, thioalkane carboxylates, mercaptoalkane sulfonates, phosphates or phosphonates come into consideration as salts. The counterions can preferably have alkyl groups or aryl groups, which can in turn advantageously be partially fluorinated. The counterions trifluoromethane sulfonate, methane sulfonate, and/or toluene sulfonate are very particularly preferred. These can also be salts in which the noble metal ions are complexed with ligands or chelate ligands, such as ethylene diamine, polyethylene glycols or thioethanol derivatives, such as 2,2-ethylen-dithiodiethanol.

Examples of preferred salts of the noble metals are copper sulfate, HAuCl₄, palladium sulfate, palladium nitrate and palladium acetate, platinum chloride, rhodium chloride, silver nitrate and silver methane sulfonate.

The bath preferably contains a content of noble metal ions between 0.1 g/l and 3 g/l, preferably between 0.1 g/l and 2 g/l, particularly preferably between 0.1 g/l and 1.8 g/l, very particularly preferably between 0.1 g/l and 1 g/l.

The noble metal is preferably added as a salt solution of a noble metal salt.

At least one complexing agent for the noble metal ions can additionally be added to the bath, in order to decrease the quantity of free noble metal ions. The precipitation and the nonspecific deposition of the noble metal on base metals is thus suppressed. Such complexing agents can simultaneously also reduce the required quantity of noble metal.

In a preferred refinement of the invention, the at least one complexing agent is an acid-stable complexing agent. In the case of silver, for example, such complexing agents are available under the name Slotoloy SNA 33 (Schlatter).

Preferred complexing agents are the organic sulfur compounds described in the publication WO 01/92606 A1: pages 7 to 9 and preferably in EP 1 285 104 B1 in paragraphs [0025] to [0027], to which reference is explicitly made here.

These organic sulfur compounds preferably have the following general formula:

X—R¹—[Z—R²]_(n)—Z—R³-y  (I)

where n=0 to 20, preferably 0 to 10, particularly preferably 0 to 5, X and Y independently of one another are each —OH, —SH, or —H, Z respectively represents a sulfur atom or an oxygen atom, and the radicals Z in the case n>1 in the formula (I) are respectively identical or different, R¹, R², and R³ independently of one another each represent an optionally substituted linear or branched alkylene group and the radicals R² in the case n>1 in the formula (I) are each identical or different. Under the presumption that Z is exclusively an oxygen atom, at least one of the groups X, Y, R¹, R² and R³ contains at least one sulfur atom.

Examples of alkylene groups are alkylene groups having 1 to 10, preferably 1 to 5 carbon atoms, e.g., methylene, ethylene, n-propylene, iso-propylene, n-butylene, iso-butylene, and tert-butylene groups. Examples of substituents of alkylene groups are —OH, —SH, —SR⁴, where R⁴ is an alkyl group having 1 to 10 carbon atoms, e.g., a methyl, ethyl, n-propyl, or iso-propyl group, —OR⁴, —NH₂, NHR⁴, and NR⁴ ₂ (wherein the two substituents R⁴ can be identical or different).

In the case that Z in formula (I) exclusively represents an oxygen atom, the sulfurous groups X and/or Y can be an SH group and/or the sulfurous groups R¹, R², and/or R³ can represent, e.g., an alkylene group which is substituted with an SH group or with an SR⁴ group.

Preferably, in formula (I)

n: is 1,

R¹, R², and R³ independently of one another are an alkylene group which has at least two carbon atoms,

and for the case that only one Z represents a sulfur atom, X and/or Y is an SH group and for the case that Z exclusively is an oxygen atom, X and Y represent an SH group.

Compounds are preferred in which Z respectively represents a sulfur atom or an oxygen atom and the groups Z are identical or different, R¹, R², and R³ independently of one another respectively represent an alkylene group which has 2 to 10 carbon atoms, n is 1 to 20, X and Y independently of one another are each —OH, —SH, or —H, and for the case that only one Z represents a sulfur atom, X and/or Y is —SH, and for the case that Z is exclusively an oxygen atom, X and Y are respectively —SH. Alkylene groups having 2 to 5 carbon atoms are preferred, e.g., ethylene n-propylene, iso-propylene, n-butylene, iso-butylene, and tert-butylene groups. Furthermore, the following organic sulfur compounds are preferred:

-   bis-(hydroxyethyl)-sulfide: HO—CH₂—CH₂—S—CH₂—CH₂—OH; -   3,6-dithiaoctanediol-1,8: HO—CH₂—CH₂—S—CH₂—CH₂—S—CH₂—CH₂—OH; -   3,6-dioxaoctanedithiol-1,8: HS—CH₂—CH₂—O—CH₂—CH₂—O—CH₂—CH₂—SH; -   3,6-dithia-1,8-dimethyloctanediol-1,8:     HO—CH(CH₃)—CH₂—S—CH₂—CH₂—S—CH₂—CH(CH₃)—OH; -   4,7-dithiadecane: H₃C—CH₂—CH₂—S—CH₂—CH₂—S—CH₂—CH₂—CH₃; -   3,6-dithiaoctane: H₃C—CH₂—S—CH₂—CH₂—S—CH₂—CH₃; -   3,6-dithiaoctane dithiol-1,8: HS—CH₂—CH₂—S—CH₂—CH₂—S—CH₂—CH₂—SH.

The at least one complexing agent is preferably added in a quantity such that the molar ratio of the complexing agent(s) to the noble metal ions (molar quantity of all complexing agents:molar quantity of noble metal ions) is at least 1, preferably 5:1 to 1:1, particularly preferably 1.5:1.

To increase the conductivity of the bath, conductive salts known from galvanic deposition can also be added. These are typically alkaline or alkaline earth salts, for example, hydroxides, chlorides, bromides, nitrates, fluoroborates, for example, potassium hydroxide, potassium chloride, sodium chloride, lithium chloride, lithium bromide or lithium hexafluoroborate. These salts are preferably provided in a quantity of 0.1 g/l to 1 g/l, preferably 0.4 g/l to 0.8 g/l, in the bath.

The bath for the currentless deposition of a metal layer is preferably a bath for the deposition of an electroless nickel layer, preferably a nickel/phosphorus and/or nickel/boron layer. This means that a nickel salt is used as the metal salt and hypophosphites and/or borates are used as the reducing agents.

In the next step, a substrate is introduced into the bath. The substrate is preferably a conductive substrate. The conductivity can also be achieved by having a conductive layer applied to the substrate.

Preferred metallic substrates are copper, bronze, aluminum and steel, in particular stainless steel.

Non-metallic substrates are, for example, plastics, such as polypropylene, polyethylene, polycarbonates, polyimides, polyamides or nylon. It is vital that they survive the conditions of the deposition. These nonmetallic substrates are preferably coated with a metallic layer.

It may be necessary to clean, degrease and/or activate the surface of the substrate before carrying out the method. This can be performed, for example, by applying a thin metal layer, for example, by a so-called “nickel strike”. This is also referred to as a nickel bond coat. A thin nickel film is deposited on the surface in this case.

The substrate is preferably a net, sieve, or lattice, preferably having a mesh width of less than 1 mm, particularly preferably less than 500 μm. However, significantly finer nets having a mesh width of less than 100 μm, preferably less than 50 μm are also possible. The wire thickness is greater than 5 μm, preferably greater than 10 μm, for example, between 50 μm and 1000 μm. The method of the invention also allows the coating of such nets, sieves, or lattices which enclose a cavity, for example, filter elements or V-filters. Because of shielding effects, such structures cannot be coated on the interior using galvanic methods.

After the introduction into the bath, a voltage is applied between the substrate and an electrode. For example, a graphite, nickel, or silver electrode can be used as the electrode. A graphite electrode is preferred.

The ratio of the area of the anode in the bath and the projected workpiece surface (shadow area) is between 1:0.5 and 1:2, preferably 1:1 (with a deviation of +/−10%).

It can be necessary to bring the bath to a specific temperature and/or a specific pH value before the application of the voltage. The bath is preferably brought to a temperature of greater than 50° C. However, the method can also be carried out at temperatures between 15° C. and 90° C. In particular in the case of a high content of noble metal ions, the temperature must not be excessively high, since otherwise the electrolyte will decompose. Temperatures of less than 70° C. are preferred, preferably between 30° C. and 70° C., particularly preferably between 50° C. and 70° C.

The pH value of the bath is preferably in the acid range below pH 6, preferably between 4.0 and 5.0, particularly preferably between 4.2 and 4.6.

A voltage is then applied between an electrode and the substrate. The electrode is connected as the anode and the substrate is connected as the cathode. It may be necessary to vary the voltage over time, for example, increasing, decreasing, or periodically.

An auxiliary electrical field is generated by the voltage. The voltage is applied in such a manner that a current density between 0.01 and 3 A/dm², preferably between 0.1 and 1 A/dm², particularly preferably between 0.1 and 0.7 A/dm², is set. This current density is significantly less than the typical current density in galvanic methods.

Surprisingly, it has been found that, using the deposition of a noble metal/metal layer from a bath for currentless deposition of the metal layer, layers can be obtained, which have particularly advantageous tribological properties. In addition, the obtained layers are very uniform, in particular even in the case of substrates in which this is not possible using solely galvanic methods. It is therefore assumed that the currentless deposition of the metal occurring in the background even induces the deposition of the noble metal/metal layer in the regions which are intrinsically unfavorable. Without the assistance by the voltage, a layer having good adhesion is not obtained.

The method is also very rapid. Thus, a layer thickness of 1 to 5 μm can already be obtained within 1 to 5 minutes. However, layer thicknesses of 0.1 μm to 5 μm are preferred, preferably 0.1 μm to 1 μm.

The obtained layers display a noble metal content of greater than 60 wt.-%, preferably greater than 80 wt.-%, particularly preferably greater than 90 wt.-%.

Still further treatment steps can follow after the deposition. This includes, for example, heat treatments to harden the layers.

In a preferred embodiment, the substrate is coated with an electroless nickel layer before carrying out the method. For this purpose, the substrate is introduced into an electroless nickel bath and an electroless nickel layer is deposited without current before carrying out the method. The adhesion of the coating deposited with current assistance on the substrate is thus significantly improved.

Such a bath contains the above-described components of an electroless nickel bath. These are at least one nickel salt and one reducing agent in the specified quantity ranges. The bath can additionally contain complexing agents for nickel ions in the specified ranges.

The speed of the currentless deposition is essentially determined by the temperature and/or by the pH value of the bath. The conditions are determined by the metal/reducing agent system used. The temperature is preferably greater than 50° C., preferably greater than 70° C., particularly preferably between 80 and 90° C., very particularly preferably between 86° C. and 90° C., e.g., at 88° C. The pH value is preferably between 4.0 and 6.0, preferably between 4.2 and 4.6, very particularly preferably at 4.4.

In a further preferred embodiment of the invention, the previously deposited electroless nickel layer has a noble metal content of up to 30 wt.-%. A bath as already described for the method of the invention is preferably used for this purpose. However, the content of noble metal ions is preferably lower by a factor of 10 to 20. This electroless nickel layer thus has a content of noble metal of up to 30 wt.-%, preferably between 1 and 10 wt.-%. The content of noble metal is between 0.01 and 0.1 g/l, preferably between 0.01 and 0.06 g/l, particularly preferably between 0.01 and 0.05 g/l or 0.01 and 0.04 g/l.

The bath also additionally contains at least one complexing agent for the noble metal ions, as already described for the other bath. Acid-resistant complexing agents are preferred. These are available in the case of silver, for example, under the designation Slotoloy SNA 33 (Schlatter).

The at least one complexing agent is preferably added in a quantity such that the molar ratio of the complexing agent(s) to the noble metal ions (molar quantity of all complexing agents:molar quantity of noble metal ions) is at least 1, preferably 10:1 to 1:1, particularly preferably 3:1.

Through the use of these complexing agents, the stability of the bath during the currentless deposition can be increased. Simultaneously, a deposition of a high content of noble metal in the deposited layer is achieved with significantly reduced content of noble metal ions. During the deposition, it may be necessary to restore the content of noble metal by further dosing.

In a preferred embodiment, this can be performed by measuring the electrochemical potential between electrolyte, i.e., bath, and a reference electrode. Upon change of the potential greater than a threshold value, the added dosing of the noble metal can be triggered.

In a preferred embodiment, the previously deposited layer contains the same noble metal as the layer deposited in the method of the invention. The adhesion of the layer deposited in steps a) to c) on the substrate is thus significantly improved. In addition, the noble metal content provides this first layer with better conductivity, which improves the deposition on this layer in steps a) to c).

Noble metal doping in the first layer also allows the use of very low field strengths during the voltage-assisted deposition, probably because of the conductivity, which is increased by the noble metal. In spite of the very low field strength in comparison to galvanic deposition, a precisely contoured deposition with good throwing power having a noble metal content of greater than 90 wt.-% can be achieved. This layer simultaneously has high abrasion resistance and a good depot effect in the case of silver.

The method for producing the first layer can also be used by itself for producing metal layers containing noble metals.

The noble metal is particularly preferably silver in at least one bath, preferably in both baths. Layers can thus be obtained which display a particularly good biocidal effect and also simultaneously have outstanding tribological properties.

In a preferred embodiment, the noble metal in both layers is silver, particularly preferably, in both layers the noble metal is silver and the metal is nickel, or nickel/phosphorus.

The invention additionally relates to a coating on a substrate, which has a noble metal/nickel layer on the surface, which has a noble metal content of greater than 60 wt.-%, preferably greater than 80 wt.-%, particularly preferably greater than 90 wt.-%.

In a further embodiment, the noble metal/nickel layer is a noble metal/nickel/phosphorus layer or noble metal/nickel/boron layer, preferably a noble metal/nickel/phosphorus layer. The content of phosphorus in the respective layer is 0.1 wt.-% to 30 wt.-%, preferably between 0.1 wt.-% and 10 wt.-%, particularly preferably 0.1 wt.-% to 3 wt.-%, in relation to the proportion of nickel and phosphorus in the respective layer.

In a preferred refinement of the invention, a nickel/noble metal layer having a noble metal content of less than 30 wt.-%, preferably between 1 wt.-% and 10 wt.-%, is arranged underneath. This layer is preferably obtained through the above-described currentless method. The layer on the surface has a thickness of 0.1 μm to 5 μm, preferably 0.1 μm to 1 μm, while the layer underneath has a thickness of 0.5 μm to 50 μm, preferably 0.5 μm to 15 μm.

Particularly low field strengths during the deposition of the noble metal/nickel layer can be implemented due to this first layer. The noble metal/nickel layer thus becomes more uniform. The formation of oxides due to excessively high current densities is also prevented. The best results with respect to the tribological stability are obtained by the structure of the coating having differing noble metal content.

In a further embodiment, the nickel/noble metal layer is a nickel/phosphorus/noble metal or nickel/boron/noble metal layer, preferably a nickel/phosphorus/noble metal layer. The content of phosphorus in the layer is 10 wt.-% to 30 wt.-% in relation to the proportion of nickel and phosphorus in the respective layer.

In a further embodiment, the noble metal is silver. The coating thus has biocidal properties. Due to the multilayered structure with the layer with the highest silver content on the surface, these properties are particularly pronounced. In addition, the multilayered structure of the coating provides particularly good adhesion on the substrate, since the different layers are structurally well matched with one another.

The coating can also contain still further layers. The two described layers preferably form the two uppermost layers of the coating. However, for example, still further electroless nickel layers or also nickel layers, for example, from a preceding actination of the surface by a nickel strike, may be provided underneath.

In a further embodiment, the coating is generated according to the method of steps a) to c), wherein previously an electroless nickel layer containing noble metal was deposited, preferably using the method also described.

The invention additionally relates to a coated substrate, obtainable according to the method of the invention or having a coating corresponding to the invention.

In a refinement of the invention, the coated substrate is a net, sieve, filter, fabric, or sponge. The substrate preferably has at least one cavity which is completely or partially enclosed by a net, sieve, filter, fabric, or sponge. Examples of such substrates are filter elements, filter inserts, or V-filters.

The invention additionally relates to the use of the coated substrate in the automotive field, sanitary field, jewelry field, drinking water field, in wastewater treatment, drinking water treatment, the filtering of fluids, in cooling water circuits, in chemical plant construction, or in electrical engineering. For example, the coated substrate can be used in an application selected from the group of filters, valves, throttle units, radial and/or filter elements, filter screens, V-filters, architecture, decoration, machines and plants of the chemical industry, and finish in the electrical industry.

Further details and features result from the following description of preferred exemplary embodiments in conjunction with the subclaims. In this case, the respective features may be implemented alone or in combination with one another. The possibilities for achieving the object are not restricted to the exemplary embodiments. Thus, for example, range specifications always comprise all intermediate values—not mentioned—and all conceivable subintervals.

In a preferred embodiment of the invention, firstly a first aqueous electroless nickel bath having a content of nickel in a range of 1 to 15 g/l and a content of reducing agent in a range from 20 to 50 g/l is provided, this bath is well stirred, and then the pH value is adjusted to a value in the range from 4.0 to 6.0 or 4.5 to 5.0.

In addition, it has proven to be particularly efficient if a pH value in the range from 4.2 to 4.6, preferably 4.4, is set. The pH value can be adjusted, for example, by adding ammonia solution or hydrochloric acid or sulfuric acid.

The nickel ions of the bath are typically provided as solutions of the salts nickel chloride, nickel sulfate, nickel carbonate, and/or nickel acetate. The nickel content is typically in a range from 3 to 10 g/l.

A phosphorus or boron compound is preferably used as the reducing agent in the bath. The reducing agent in the bath is preferably a hypophosphite. The reducing agent is very particularly preferably sodium hypophosphite and/or potassium hypophosphite.

Dimethyl aminoborane, diethyl aminoborane, or sodium borohydride can be used as boron compound. The reducing agent is normally provided in a concentration in a range of 32 to 42 g/l in the bath.

The bath optionally also contains at least one complexing agent, which is selected in particular from the group of monocarboxylic acids, dicarboxylic acids, hydroxycarboxylic acids, ammonia, and alkanolamines. The complexing agent is provided in a concentration in a range from 1 to 15 g/l in the bath. Complexing agents complex nickel ions and thus prevent excessively high concentrations of free nickel ions. The solution is thus stabilized and the precipitation of, for example, nickel phosphite is suppressed.

The bath optionally also contains at least one accelerator, which is selected in particular from the group containing fluorides, borides, and/or anions of monocarboxylic and dicarboxylic acids. The accelerator is typically provided in a concentration in a range from 0.001 to 1 g/l in the bath. Accelerators can activate hypophosphite ions, for example, and thus accelerate the deposition.

Typical nickel baths can also contain at least one stabilizer, which is selected in particular from the group of lead, tin, arsenic, molybdenum, cadmium, and thallium ions and/or thiourea. The stabilizer is typically provided in a concentration in a range from 0.01 to 250 mg/l in the bath. Stabilizers can prevent the decomposition of the solution by masking catalytically active reaction seeds.

The bath typically also contains at least one pH buffer, which is a sodium salt of a complexing agent and/or the associated corresponding acid in particular. The buffer is typically provided in a concentration in a range from 0.5 to 30 g/l in the bath.

The bath optionally also contains at least one pH regulator, which is selected in particular from the group of sulfuric acid, hydrochloric acid, sodium hydroxide, sodium carbonate, and/or ammonia. The pH regulator is advantageously provided in a concentration in a range from 1 to 30 g/l in the bath. pH regulators allow the pH value of the bath to be readjusted.

The bath can also contain at least one wetting agent, which is selected in particular from the group of ionogenic and/or non-ionogenic surfactants. The wetting agent is preferably provided in a concentration in a range from 0.001 to 1 g/l in the bath. Wetting agents increase the wetting of the surface to be nickel plated and allow the production of very uniform layers.

This first electroless nickel bath additionally contains 0.01 to 0.1 g/l, preferably between 0.01 and 0.06 g/l, particularly preferably between 0.01 and 0.05 g/l or 0.01 and 0.04 g/l of silver or silver ions. The silver is preferably added to the bath as a solution of a silver salt. It can be necessary to add the solution very slowly.

Silver nitrate, silver acetate, or a silver salt of a sulfonic acid or thiocarboxylic acid, for example, silver methane sulfonate, can be used as the silver salt.

The bath preferably additionally contains at least one complexing agent for silver ions, in order to stabilize the bath. Acid-resistant complexing agents are preferred. These are available in the case of silver, for example, under the designation Slotoloy SNA 33 (Schlatter). The organic sulfur compounds preferred as the complexing agents were already described.

The at least one complexing agent is preferably added in a quantity such that the molar ratio of the complexing agent(s) to the noble metal ions (molar quantity of all complexing agents:molar quantity of noble metal ions) is at least 1, preferably 10:1 to 1:1, particularly preferably 3:1.

A substrate is introduced into this first bath and, at a temperature between 80° C. and 95° C., preferably 80° C. and 90° C., preferably between 86° C. and 90° C., particularly preferably at 88° C., an electroless nickel layer having a silver content of up to 30 wt.-%, preferably between 1 to 10 wt.-%, is deposited. The thickness of this layer is between 0.5 μm and 50 μm, preferably between 0.5 μm and 15 μm.

During the deposition, it may be necessary to resupplement the silver consumed by the deposition. The dosing can be performed by individual additions or continuously.

In a preferred embodiment, the additional dosing is controlled by the measurement of the electrochemical potential between electrolyte and a reference electrode.

It can be advantageous for the surface of the substrate to be pretreated, for example, by a nickel strike. Thus, for example, oxide layers on metallic substrates are removed. Such a nickel strike can arise, for example, by treating the surface using a mixture of inorganic acids, nickel chloride, citric acid, and acetic acid while applying a voltage. The surface is thus cleaned and simultaneously a thin nickel layer, which is 10 nm to 1 μm thick, is deposited on the substrate. Such a nickel strike is preferably carried out in the case of metal substrates. The compositions are known to a person skilled in the art.

After the deposition of the first layer, the substrate is introduced into a second bath. This bath also corresponds to a bath for the currentless deposition of nickel, which also additionally contains silver.

With respect to the silver ions, the bath preferably contains a content of silver between 0.1 g/l and 3 g/l, more preferably between 0.1 g/l and 2 g/l, particularly preferably between 0.1 and 1.8 g/l, very particularly preferably between 0.1 and 1 g/l.

The silver is preferably added as a salt solution of a silver salt.

The bath can also contain at least one conductive salt. These are typically inorganic salts. These are normally alkaline or alkaline earth salts, for example, hydroxides, chlorides, bromides, nitrates, fluoroborates, for example, potassium hydroxide, potassium chloride, sodium chloride, lithium chloride, lithium bromide, or lithium hexafluoroborate.

In a preferred refinement of the invention, the conductive salts are added in the quantity for a content of 0.1 to 1 g/l, preferably between 0.4 to 0.8 g/l.

Such conductive salts are also available as commercial solutions. An example of such a batch solution is Arguna CF (Umicore).

It may be necessary to briefly clean the substrate between the two baths, for example, by plunging it into a cleansing bath, for example, water. Preferably, no cleaning steps are carried out between the two baths. The substrates can be transferred directly from one bath into the other bath.

A voltage is applied between the substrate and an electrode in this bath. The electrode is connected as the anode and the substrate is connected as the cathode.

An auxiliary electrical field is generated by the voltage. The voltage is applied in such a manner that a current density between 0.01 and 3 A/dm², preferably between 0.1 and 1 A/dm², particularly preferably between 0.1 and 0.7 A/dm², is set. This current density is significantly less than the typical current density in galvanic methods.

The deposition is preferably carried out at a temperature between 21° C. and 90° C. The temperature is typically less than the transition temperature for the nickel bath, i.e., less than 70° C., preferably between 30° C. and 70° C., particularly preferably between 50° C. and 70° C. A higher content of silver ions in the electrolyte is thus possible, without the electrolyte decomposing.

The pH value of the bath is preferably in the acid range below pH 6, preferably between 4.0 and 5.0, particularly preferably between 4.2 and 4.6.

With the application of the voltage, a layer, which consists of a majority of silver, preferably having a silver content of greater than 60 wt.-%, more preferably having a silver content of greater than 80 wt.-%, very particularly preferably of greater than 90 wt.-%, begins to be deposited on the substrate.

This deposition is preferably carried out for up to 5 min. Layers having a thickness of up to 5 μm, preferably up to 1 μm, particularly preferably between 0.1 μm and 1 μm are thus obtained.

It is assumed that the first deposited silver-containing nickel layer significantly improves the adhesion of this second layer. Simultaneously, the better conductivity of the nickel layer containing noble metal in combination with the simultaneously occurring currentless deposition promotes uniform deposition even in substrates which cannot be coated solely using galvanic methods.

Such substrates are in particular nets or sieves which are used in filter elements. These are cavities which are completely or partially enclosed by such sieve or net surfaces. Because of shielding effects, these substrates cannot be uniformly coated on the interior using solely galvanic methods. Using the method of the invention it is possible to apply a uniform layer to both the interior and also the exterior of the substrate.

The exemplary embodiments are schematically shown in the figures. Identical reference numerals in the individual figures designate identical or functionally identical elements or elements which correspond to one another with respect to their functions.

IN THE DRAWING

FIG. 1 shows a schematic illustration of a filter element;

FIG. 2 shows a schematic illustration of a preferred embodiment of the method of the invention;

FIG. 3 shows a typical structure of a layer of the invention;

FIG. 4A shows light-microscope pictures of an uncoated sieve or screen;

FIG. 4B shows light-microscope pictures of an uncoated sieve or screen;

FIG. 5A shows light-microscope pictures of the sieve or screen from FIGS. 4A and 4B after the coating;

FIG. 5B shows light-microscope pictures of the sieve or screen from FIGS. 4A and 4B after the coating.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a schematic illustration of a filter element, which can be coated using the method according to the invention, in particular with a nickel/silver layer. The filter element consists of a cylindrical filter body 10. This consists of a net-like structure, which forms a cylinder. The two ends of the cylinder are closed using caps 12. Connection openings can also be attached to these caps. The filter element can also have fasteners for seals. Together with the caps 12, the filter body encloses a cavity. Using the method according to the invention, it is possible to provide the filter body on the interior and on the exterior with a uniform, tribologically stable coating. In the case of silver-containing coatings, the filter element also has biocidal effects. The preferred material for the filter element is stainless steel, e.g., 1.4404, 1.4301, 1.4571 material designation according to AISI.

FIG. 2 shows the schematic sequence of a preferred embodiment of the invention. The substrate surface is first cleaned and optionally pretreated using a nickel strike (200).

In the next step (210), the substrate is introduced into a first bath. This bath is a bath for the currentless deposition of a metal/noble metal layer, as already described.

A first layer is deposited (220) without current on the substrate in the first bath. This layer is a metal/noble metal layer, wherein the layer has a noble metal content of up to 30 wt.-%. The deposition is performed without current, in order to obtain a uniform coating of the substrate.

After a layer of sufficient thickness has been obtained, the substrate, optionally after cleaning steps, is introduced into a second bath (230). This bath is—as already described—a bath for the currentless deposition of a metal layer which additionally contains noble metal ions.

In the next step, a voltage is applied between the substrate and an electrode (240). Deposition of a noble metal/metal layer thus occurs. This is a layer which has a content of noble metal of greater than 60 wt.-%.

The coating is carried out until a layer having a thickness of up to 5 μm, preferably up to 1 μm, is obtained.

In a preferred embodiment, the metal layer stands for a nickel/phosphorus layer and the noble metal stands for silver. Using the method of the invention, layers can thus be obtained which consist of a nickel/phosphorus/silver layer and a silver/nickel/phosphorus layer thereon, each ordered according to the proportion in the layer. This combination of layers displays a particularly advantageous biocidal effect while simultaneously having very good tribological properties. Filter elements coated in this manner are significantly more durable than filter elements only coated without current or those having coatings made of pure silver. The method is also significantly more cost-effective to carry out than currentless coating using the pure noble metal.

The substrate coated in this manner having at least two layers displays particularly high tribological stability and is very well suitable for filter elements in fluids, for example, in cooling water circuits.

FIG. 3 shows a structure of a preferred embodiment of the invention. An electroless nickel/phosphorus/silver layer (32) having a silver content of <10 wt.-% with a thickness between 0.5 and 15 μm is arranged on a substrate (34). A silver/nickel/phosphorus cover layer having a silver content of >90 wt.-% and a thickness between 0.1 μm and 1 μm is located thereon.

FIG. 4 shows pictures of an uncoated sieve/screen/filter element as shown in FIG. 1 at varying magnification. The scale is 200 μm in each case. The thickness of the horizontal wires is approximately 150 μm and 160 μm. The measured mesh widths are between 85 μm and 143 μm.

FIG. 5 shows light-microscope pictures of the same sieve/screen/filter element from FIG. 4 after application of a coating according to the invention having a nickel/silver and a silver/nickel layer, as shown in FIG. 3. The scale is 200 μm in each case. The thickness of the horizontal wires is now approximately 160 μm and 170 μm. The measured mesh widths are between 40 μm and 120 μm. Uniform coating of the substrate can be clearly recognized. Such coated filters display a biocidal effect during use in cooling circuits over several weeks, without detachment of the coating occurring.

Example 1. Currentless Deposition of a Silver-Containing Nickel Layer.

Silver in the form of silver methane sulfonate corresponding to a silver proportion of 0.025 g/l and an acid-resistant silver complexing agent (e.g. Slotoloy SNA 33, producer Schlatter) in a quantity corresponding to 0.2 ml/l are added to an above-described electroless nickel bath. The pH value of the bath is adjusted to 4.4 using H₂SO₄. The bath is then heated to 88° C. and a substrate made of stainless steel is introduced. The consumption of silver ions is compensated by continuous additional dosing of silver methane sulfonate. This can be performed such that the dosing is controlled via the measurement of the electrochemical potential between electrolyte and a reference electrode.

Under these conditions, a nonporous electroless nickel silver alloy layer, which adheres well to stainless steel, having a silver content of 1-10 wt.-% is deposited.

A higher silver content cannot be achieved like this, since the silver content cannot be increased further in relation to the nickel content in the bath. Higher contents of silver result in complete destabilization of the electrolyte (spontaneous self precipitation) at a temperature from approximately 70° C. It is not possible to avoid this effect by increasing the contents of stabilizers or complexing agents, without the nickelphosphorus reduction coming to a stop. The ranges of noble metal and noble metal content of the layer specified in DE 10 2006 020 988 A1 are thus not possible for silver. The substrate coated in this manner was then provided according to the following specifications with a silver/nickel layer.

2. Voltage-Assisted Deposition of a Silver/Nickel Layer.

Silver in the form of silver methane sulfonate corresponding to a silver proportion in the bath of 0.6 g/l, an acid-resistant silver complexing agent (e.g., Slotoloy SNA 33, producer Schlatter) in the quantity corresponding to 4 ml/l in relation to the bath and a commercial galvanic silver batch salt, only the conductive salt, (e.g., Arguna CF, Umicore) corresponding to a quantity of conductive salt in the bath of 0.65 g/l, are added to an above-described electroless nickel bath. The proportion of silver ions can thus be increased in comparison to the first bath by this composition such that no spontaneous reaction occurs.

Graphite, nickel, or silver electrodes, preferably graphite electrodes, are introduced into the bath and an electrical voltage is applied, wherein the introduced electrodes are connected as the anode and the workpiece/substrate is connected as the cathode. The area of the anode corresponds to approximately the projected workpiece surface (i.e., a ratio of 1:1). The current density of the auxiliary field is between 0.1 and 1 A/dm². This is a significantly lower current density than in typical galvanic silver depositions. This is normally at 30 to 100 A/dm².

A precisely contoured deposition, having good throwing power, of a silver-nickel alloy having a silver content of >90 wt.-% can thus be achieved. The layer, or the combination of the nickel/silver and silver/nickel layers, has a high abrasion resistance and depot effect with respect to the silver.

The coating obtained in the two-step method also still displayed a biocidal effect and outstanding stability after continuous operation over several months in a cooling circuit.

LIST OF REFERENCE NUMERALS

-   10 filter body -   12 closure caps -   200 pretreatment of the substrate surface -   210 introduction into a first bath -   220 currentless deposition in the first bath -   230 introduction into a second bath -   240 voltage-assisted deposition in the second bath -   30 Ag—Ni cover layer -   32 NiP—Ag alloy -   34 substrate

CITED LITERATURE

-   DE 10 2006 020 988 A1 -   GB 1 222 969 -   WO 01/92606 A1 -   EP 1 285 104 B1 

1.-18. (canceled)
 19. A method for depositing metal layers on a substrate, comprising the following steps: a) providing a first bath for the currentless deposition of a metal layer, which additionally contains at least one type of noble metal ions; b) introducing the substrate into the first bath; c) currentlessly depositing metal and noble metal ions from the first bath onto the substrate to form a first metal/noble metal layer having a noble metal content of up to 30 wt.-%; d) introducing the substrate into a second bath for the currentless deposition of a metal layer, which additionally contains at least one type of noble metal ions, or leaving the substrate in the first bath; e) applying a voltage between an anode, which is immersed in the bath, and the substrate, which is connected as a cathode; and f) galvanically depositing a second noble metal/metal layer on the first layer on the substrate, wherein the second noble metal/metal layer has a noble metal content of greater than 60 wt.-%.
 20. The method as claimed in claim 19, characterized in that at least one of the baths for the currentless deposition of a metal layer is a bath for the deposition of an electroless nickel layer.
 21. The method as claimed in claim 19, characterized in that the noble metal ions are selected from the group containing silver, gold, palladium, platinum, rhodium, and copper.
 22. The method as claimed in claim 19, characterized in that the voltage is applied at a bath temperature of less than 70° C.
 23. The method as claimed in claim 19, characterized in that at least one of the baths also contains at least one complexing agent for the noble metal ions.
 24. The method as claimed in claim 19, characterized in that the content of noble metal ions in the bath is between 0.1 and 3 g/l.
 25. The method as claimed in claim 19, characterized in that a voltage is applied corresponding to a current density between 0.01 and 3 A/dm².
 26. The method as claimed in claim 19, characterized in that the first bath for the currentless deposition of a metal layer is a bath for the deposition of an electroless nickel layer.
 27. The method as claimed in claim 26, characterized in that the noble metal in both layers is silver.
 28. A coating for a substrate, characterized in that the coating has a noble metal/nickel/phosphorus layer or a noble metal/nickel/boron layer, which has a noble metal content of greater than 90 wt.-% at the surface.
 29. The coating as claimed in claim 28, characterized in that the noble metal is silver.
 30. A coated substrate obtained by a method as claimed in claim
 19. 31. The coated substrate as claimed in claim 30, characterized in that the substrate is a net, sieve, filter, fabric, or sponge.
 32. A substrate having formed thereon a coating as claimed in claim
 28. 33. The substrate as claimed in claim 32, characterized in that the substrate is a net, sieve, filter, fabric, or sponge. 